The field of computer generated imaging has evolved to include techniques for printing full-color images on various receivers (e.g., print media such as paper and transparent films). Various types of data directed or programmable printing machines (e.g., thermal printers and laser printers) have been devised to accomplish this full-color printing. One characteristic which many of these machines have in common is that a color image is made by three successive printing steps which involve the deposition of three printing colors on top of each other. For example, in the case of thermal printers, the colors are yellow, cyan and magenta.
An inherent difficulty in any multi-stage full-color printing is color-registration. In a typical full-color printing operation, a first primary color image is applied to the receiver. Then second and third primary color images are applied to the receiver. Typically the receiver is moved back and forth as each color is applied. In many instances the receiver does not return to precisely the same position under a print head after each back and forth movement. This dislocation of the receiver results in a misregistration of the three primary color images. Misregistration of the three images is discernable to a viewer as a defect.
A great deal of attention and effort has been applied to the problem of achieving accurate color registration in printing. Many elaborate systems have been created to carefully control the position of the receiver within a printer.
In one known technique, detectors are employed to determine if the receiver is properly positioned. To correct a positioning error, various devices have been used to move the receiver into a desired location under a print head of a printer. Printers which have receiver-position correction features are typically quite complex, expensive to build and difficult to maintain. As a result, printers with these features are typically found only in large scale printing operations such as the printing of magazines, or other high volume commercial items.
Another known technique for controlling color registration utilizes lock-clamping of the receiver to a transport device, such as a roller platen. When the receiver is locked on a platen, its position through successive printing steps is more predictable. These types of lock-clamping devices introduce undesirable complexities to the operation of printers. For example, the steps of clamping and unclamping print receiver often requires a printing operation to stop and start. This increases print cycle time.
Printers that generate full-color images from computerized data are particularly desirable when used in conjunction with relatively small personal computers. One ideal application for a machine such as a full-color thermal printer or laser printer is in an office or laboratory setting where office workers operate the printers. In this setting it is desirable that the printers be small, inexpensive and easy to maintain. Prior art techniques for controlling color registration, lock-clamping and re-positioning of receivers, add substantially to the cost of these printers. In many cases printers employing such prior art techniques are not practical in an office or laboratory application because of high cost, low speed, or lack of simplicity.
It is desirable to have a relatively simple full-color printing system which provides accurate alignment of colors on a receiver at relatively low cost and high speed.